1. Field of the Invention
The present invention relates to an optical modulator, a semiconductor laser device equipped with an optical modulator, and an optical communications system, and more particularly, to an optical modulator involving little distortion in the waveform of a signal after transmission thereof, a semiconductor laser device equipped with the optical modulator, and an optical communications system for transmitting an optical signal over a long distance through use of the semiconductor laser device.
2. Background Art
An improvement in the performance of a semiconductor laser and an improvement in the yield of a semiconductor laser required for inexpensively manufacturing a semiconductor laser are important in realizing widespread proliferation of a public communication network that uses an optical fiber.
High-speed modulation of a laser beam for coping with an increase in the volume of information transmitted is particularly indispensable for improving the performance of a semiconductor laser. In order to enable long-distance transmission of an optical signal while limiting variation in wavelength, which arises at the time of modulation, an external modulation system is adopted for modulating a laser beam at high speed. In this system, light emitted from a semiconductor laser usually has a predetermined intensity and is passed through an optical modulator capable of inducing and interrupting transmission of light, to thereby modulate the light.
An electroabsorption modulator (EAM) is used as an optical modulator for use with the external modulation system. EAMs can be roughly divided into two types: that is, an FAM using a single thick light-absorption layer, and an EAM employing a multiple quantum well (MQW) structure formed by means of stacking thin quantum well layers, each quantum well layer being capable of forming excitons at room temperature. The former type of EAM effects extinction by utilization of variation in an absorption spectrum due to the Franz-Keldysh effect, and the latter type of EAM effects extinction by utilization of variation in absorption spectrum due to the Stark effect.
In an optical modulator, absorption of a laser beam is changed in accordance with a voltage applied to the optical modulator. For this reason, when a modulation signal voltage is applied to a high-frequency electric circuit connected to an optical modulator, a laser beam emitted from an exit end face of an optical modulator is subjected to intensity modulation in accordance with the modulation signal voltage.
In a case where a light absorption layer of an optical modulator is constituted of an MQW structure, a high extinction ratio (the ratio between the amount of light transmitted during an ON operation of the optical modulator and the amount of light transmitted during an OFF operation of the optical modulator) is attained. Therefore, a light absorption layer having an MQW structure is usually used for high-speed transmission.
FIG. 27 is a cross-sectional view showing a conventional optical modulator. In FIG. 27, reference numeral 1 designates an n-type InP substrate (hereinafter an n-type is depicted as xe2x80x9cn-,xe2x80x9d and a p-type is depicted as xe2x80x9cp-xe2x80x9d); 2 designates an n-type optical confinement layer formed of n-InGaAsP; 3 designates a light absorption layer formed of InGaAsP; 4 designates a p-type optical confinement layer formed of p-InGaAsP; 5 designates an Fe-doped InP embedded layer; 6 designates an n-InP embedded layer; 7 designates a p-type cladding layer formed of p-InGaAsP; 8 designates a p-InGaAs contact layer; 9 designates a SiO2 dielectric film; 10 designates a Ti/Au surface electrode; 11 designates an Au surface-plating layer; 12 designates an Au/Ge/Ti/Pt/Ti/Pt/Au underside electrode; 13 designates an underside plating layer; and 14 designates an optical modulator (EAM).
FIG. 28 is a cross-sectional view showing the light absorption layer 3 of conventional type. In FIG. 28, reference numeral 3a designates a quantum well layer (hereinafter referred to simply as a xe2x80x9cwell layerxe2x80x9d), and 3b designates a barrier layer. In the MQW structure of the light absorption layer 3, all ten well layers 3a are of equal thickness, and all nine barrier layers 3b are of equal thickness.
FIG. 29 is an energy diagram of the light absorption layer 3. In FIG. 29, reference symbol Ec denotes the conduction band; and Ev denotes the valence band.
FIG. 30 is a graph showing an absorption spectrum of the optical modulator 14.
In order to achieve efficient extinction by making changes in the absorption spectrum of each of the well layers 3a of the conventional light absorption layer 3 having an MQW structure, usually all the well layers 3a are made equal in band-gap wavelength and thickness. An absorption spectrum xe2x80x98Axe2x80x99 shown in FIG. 30 represents a typical absorption spectrum of the optical modulator (EAM) 14 equipped with the light absorption layer 3 having an MQW structure.
If an electric field is applied to the MQW structure by application of a bias voltage to the optical modulator 14, the absorption spectrum xe2x80x98Axe2x80x99 is changed to an absorption spectrum xe2x80x98Bxe2x80x99. In a case where the wavelength of laser incident light is set to xcex0, the absorption coefficient a relative to the incident light is changed by the bias voltage. The absorption coefficient xcex1 at the wavelength xcex0 of the incident light is changed by application of a bias voltage, to thereby turn laser light on and off. The optical modulator (EAM) 14 operates on the basis of the principle mentioned above.
Eq. (1) of the Kramers-Kronig relation applies to the amount of changes in an absorption spectrum (xcex94xcex1) in response to a change in the bias voltage and the amount of changes in refractive index (xcex94n).                               Δ          ⁢                      xe2x80x83                    ⁢                      n            ⁡                          (                              λ                0                            )                                      =                                            λ              0              2                                      2              ⁢                              π                2                                              ⁢                                    lim                              ϵ                ->                0                                      ⁢                                          (                                                      ∫                    0                                          λ0                      -                      ϵ                                                        ⁢                                      xe2x80x83                                    ⁢                                      +                                          ∫                                              λ0                        +                        ϵ                                            ∞                                                                      )                            ⁢                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  α                                                                      λ                    0                    2                                    -                                      λ                    2                                                              ⁢                              xe2x80x83                            ⁢                              ⅆ                λ                                                                        (        1        )            
During the course of an optical modulation operation, the refractive index of the optical modulator is changed in accordance with variations in the absorption spectrum of the light absorption layer 3 of the optical modulator 14. Eventually, the wavelength of the light emitted from the optical modulator 14 is changed. In other words, a chirping phenomenon arises.
FIGS. 31A and 31B are graphs showing the relationship between chronological change in light intensity and a chirping phenomenon.
By reference to FIGS. 31A and 31B, the relationship will now be described. When optical modulation of an optical signal is actually performed, the voltage applied to the optical modulator 14 assumes a value of 0V at a point at which light of the highest intensity is transmitted (i.e., a point indicated by P0 in FIG. 31A). The voltage applied to the optical modulator 14 assumes a voltage of about xe2x88x921V even at a point at which sufficiently low intensity of light is transmitted (i.e., a point indicated by P1 in FIG. 31A). Within the range between the point P0 and the point P1, an xcex1 parameter usually assumes a positive value; that is, variation in the refractive index of an optical modulator assumes a positive value. When the intensity of light is increased, as shown in FIG. 31A, a negative variation arises in the wavelength of incident laser light, as shown in FIG. 31B. In contrast, when the intensity of light is decreased as shown in FIG. 31A, a positive variation arises in the wavelength of laser incident light, as shown in FIG. 31B. Such a positive variation in wavelength is called a positive xe2x80x9cchirpxe2x80x9d phenomenon.
An optical fiber; which serves as a transmission line, has a wavelength dispersive characteristic such that the group velocity of light differs according to wavelength. For this reason, if a positive chirp phenomenon arises, the waveform of light is deteriorated after transmission thereof, because of the wavelength dispersive characteristic of the optical fiber. As a result, the waveform of received optical data is greatly impaired. In order to improve the reliability of received data, the deterioration in waveform of the received data must be compensated by repetition of transmission/receipt of an optical signal at a short interval of distance. Thus, an optical communications system inevitably becomes expensive.
As the xcex1 parameter assumes a greater positive value; that is, as a variation in the refractive index of the optical modulator assumes a greater positive value, the deterioration in waveform of an optical signal after transmission thereof becomes greater. For this reason, in addition to application of a modulation voltage, there is employed a method of applying a negative DC bias voltage to the optical modulator 14, in advance, in order to use the optical modulator 13 within the range where the xcex1 parameter assumes a small value. However, the method encounters problems; for example, a modulated waveform being deteriorated for reasons of an increase in the loss of absorption of the optical modulator 14 or an increase in the ratio of change in absorption coefficient; and an extinction ratio dropping because of significant collapse of excitons induced by a strong electric field. Therefore, practical use of an optical modulator within a range where the xcex1 parameter is sufficiently small is considerably difficult.
The present invention has been conceived to solve the drawbacks of the background art set forth.
The object of the present invention is to provide an optical modulator and a semiconductor laser device having the optical modulator, both reducing variations in the refractive index of an optical modulator or making variations negative without involvement of an increase in loss or a decrease in extinction ratio. The present invention further provides an optical communications system capable of increasing an interval of distance at which modulated light is transmitted, by use of the optical modulator and the semiconductor laser device having an optical modulator.
For reference, Japanese Patent Application Laid-Open No.231272/1999 describes an optical modulator having a pin structure for enabling optical transmission of an optical signal over a long distance without distortion of the waveform of the optical signal. An MQW modulation layer is formed by alternately stacking a quantum well layer and a barrier layer, where the quantum well layer includes quantum dots whose material is smaller in energy band-gap than a material constituting a quantum well layer. The pin structure is formed by means of interposing an MQW modulation layer between an intrinsic n-type cladding layer and an intrinsic p-type cladding layer.
Further, the collection of proceedings relating to the 45th Allied 15 Symposium of Applied Physics (29a-SZL-24, at Tokyo Engineering University, March, 1998) includes a paper describing a multiple quantum well electric field absorption type optical modulator, in which a blue chirp arises as a result a sandwiched thin buffer layer.
According to one aspect of the present invention, an optical modulator comprises a semiconductor substrate of a first conductivity type, a light absorption layer laid on the semiconductor substrate, and a semiconductor cladding layer of second conductivity type laid on the light absorption layer. The light absorption layer has a multiple quantum well structure. The multiple quantum well structure comprises a first well layer and a plurality of second well layers, and the peak wavelength of absorption spectrum of the second well layer is shorter than the peak wavelength of absorption spectrum of the first well layer.
In another aspect, in the optical modulator, preferably the first well layer positioned closest to the semiconductor substrate has the longest peak wavelength of absorption spectrum.
In another aspect, in the optical modulator, preferably the first well layer is thicker than the second well layer.
In another aspect, in the optical modulator, preferably the band-gap wavelength of the first well layer is longer than the band-gap wavelength of the second well layer.
In another aspect, in the optical modulator, a plurality of first well layers may be employed, and a barrier layer sandwiched between the first well layers has a thickness at which carriers bind together within the space between the first well layers by means of an electric field corresponding to an extinction voltage.
In another aspect, in the optical modulator, preferably the peak wavelength of an absorption spectrum of the first well layer is longer than the peak wavelength of an absorption spectrum of the second well layer by 10 nm or more.
In another aspect, in the optical modulator, preferably the barrier layer interposed between the first and second well layers is thicker than the barrier layer sandwiched between the second well layers.
According to another aspect, a semiconductor laser device is provided which comprises a semiconductor laser and one of the optical modulators as stated above integrally manufactured on a same semiconductor substrate.
According to another aspect, an optical communications system is provided which comprises one of the optical modulators as stated above or the semiconductor laser device equipped with an optical modulator.
Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings.